1. Field of the Invention
This invention relates to optically-based ultrasonic sensors, including optical communicators and displacement/velocity remote sensing methods, which are based on time delay interferometry.
2. Description of the Related Art
Ultrasonic acoustic waves are commonly used to probe a variety of materials, especially for thickness gauging and flaw detection. The conventional approach is to generate the waves with a contact piezoelectric transducer. The launched waves propagate through the material, reflecting from interfaces between layers of different materials in thickness gauging applications, or from internal features in flaw detection applications. The scattered waves propagate back to the surface of the material, causing its surface to vibrate at the ultrasound frequency. The surface vibration is generally detected with a contact piezoelectric transducer similar to the one used to generate the ultrasonic waves.
Optical detection techniques, such as those described by C. B. Scruby and L. E. Drain in Laser Ultrasonics, Techniques and Applications, Adam Hilger, New York (1990), pages 325-350, can be used instead of piezoelectric transducers to detect a surface's displacement. Generally, a laser beam illuminates the surface so that when it vibrates, a phase shift is imparted to the reflected beam. The reflected beam is interfered with a reference beam that originates from the same laser source as the reflected beam. The amplitude and frequency of the intensity fluctuations of the interfering beams correspond to the surface's motion and can be detected with a photodetector.
A "velocity-sensing", or time-delay, interferometer produces an output that is proportional to the velocity (or differential displacement), rather than the displacement, of the moving surface. Time-delay interferometric configurations (described in the Scruby et al. reference, pp. 123-127) provide one technique for doing this. In time-delay interferometry, a probe beam is reflected from the target surface, and the reflected beam is then split into two beams. One of the two beams is time delayed with respect to the other, i.e. it traverses a longer distance. The two beams are recombined along a common axis and propagate collinearly towards a photodetector where they constructively or destructively interfere: the light intensity at the photodetector is proportional to the velocity (or differential displacement) of the target surface. If the surface being probed is diffusely reflecting or scattering, a "speckle" field distribution will be formed on the photodetector. Since the reflected beam is interfered with a time-delayed replica of itself, the wavefronts of the two interfering beams are substantially matched. Consequently, a phase shift in one arm of the interferometer is common to all speckles, and all speckles can be detected optimally. Unlike a conventional interferometer, which has a flat frequency response with respect to phase shifts, a time delay interferometer has a bandpass-type of response in which low frequency vibrations (below ultrasonic frequencies) are suppressed.
There are two problems with this time-delay, or velocity, sensing system, however. First, the speckles are generally so numerous and small that the time delayed arm of the interferometer cannot be easily aligned with the other arm. Unfortunately, introducing a time delay is most easily accomplished with a multimode fiber which further increases the number of speckles and scrambles their locations, making speckle registration very difficult. The second problem is that the time delay must be held constant and stabilized in quadrature, which is a requirement for homodyne detection. If the path length difference that causes the time delay between the beams is not maintained to within a fraction of a wavelength, the sensitivity and temporal resolution of the system will be greatly reduced. As a result, both time-delay and conventional interferometers must typically employ active stabilization techniques. In industrial environments, however, the effectiveness of these techniques is reduced by noise-induced vibrations.
A velocity, or differential, sensing interferometer system which circumvents some of these problems is described in a copending application to P. V. Mitchell, D. M. Pepper, T. R. O'Meara, M. B. Klein, S. W. McCahon and G. J. Dunning, "A System and Method for Detecting Ultrasound Using Time-Delay Interferometry" (Ser. No. 08/481,673, filed Jun. 7, 1995). This system includes a time delay interferometer (TDI), a fringe processing unit (FPU) and a processor that extracts information from the output signal of the FPU. The FPU is preferably a non-steady-state photo-electromotive-force (NSS-photo-EMF) detector made from a photoconductor. Various NSS-photo-EMF detectors are described by M. P. Petrov et al. in "Non-steady-state photo-electromotive-force induced by dynamic gratings in partially compensated photoconductors", Journal of Applied Physics, Vol. 68, No. 5 (1990), pp. 2216-2225, and by S. I. Stepanov et al. in "Measuring vibration amplitudes in the picometer range using moving light gratings in photoconductive GaAs:Cr", Optics Letters, Vol. 15, No. 21 (1990), pp. 1239-1241.
With an NSS-photo-EMF detector, a space charge grating is formed in the photoconductor in response to alternating light and dark interference fringes which move in accordance with the velocity of the surface being examined. If the interference fringes move at a rate that is faster than the response time of the space charge grating, a net current flows through the photoconductor. An NSS-photo-EMF detector is sensitive to the frequency of the overall fringe pattern motion, rather than the exact shape of the fringe pattern as is the case with conventional detectors. When it is combined with a TDI, a system is produced that is relatively insensitive to the surface roughness of the target. Nevertheless, this approach requires lasers with long coherence lengths (on the order of meters) in order to coherently detect the desired high frequency modulated information produced by the vibrating surface. This means that a rather costly laser must be used, such as a single mode diode pumped solid state laser or a stabilized diode laser.
A dual time delay interferometer scheme that relaxes this long coherence length requirement is described by D. J. Erskine and N. C. Holmes in "White-light velocimetry," Nature, pp. 317-320, Sep. 28, 1995. It uses two TDIs and enables sensing of white light encoded information even over an aberrated path. The first interferometer imprints the illumination light with a coherent echo having a delay time given by the path length difference of its two arms. Light passing through the first interferometer and reflected from the target is observed by a second interferometer having its own characteristic delay time. As long as the speed of light times the difference in the delay times (i.e. the difference between the respective path length differences of the two interferometers) is less than the coherence length of the illumination light, partial fringes will be formed. This permits light sources with relatively short coherence lengths (e.g. white light) to be used as the illumination light. On the other hand, the dual-interferometer described by Erskine and Holmes is bulky (which severely limits its use, practicality and portability), since it must perfectly image and register the light reflected from the target onto a time delayed replica of itself, so that their wavefronts are spatially matched. Furthermore, this device requires that its optical components be interferometrically stable, and it cannot be used for phased array detection of objects.
The white light approach demands precision alignment and perfect imaging of the time-delay and through-beams in order to realize fringes; the precise alignment matches the two patterns, and the imaging maintains a near-field, phase-only field distribution, thereby avoiding speckle problems. It also demands interferometric stability and quadrature maintenance of the time-delay and through-beams to generate the desired fringe pattern, and requires that the incident beam be spatially coherent as it probes the workpiece; the precision imaging in the initial time-delay interferometer follows from this requirement.
The prior white light approach requires a 2-dimensional recording medium to evaluate and process the diagnostic output (such as a film plane or a video camera, in the case of real-time implementation), as well as precision image-preserving relay optical systems. These systems must possess high-resolution, flat-field, large field-of-view, distortion-free imaging capabilities to enable ultrasonic sensing. Moreover, in general the two imaging systems must be identical in terms of magnification and registration, and they must possess a large depth-of-focus when probing three-dimensional (i.e., non-planar) structures.